How to use the speed converter
- Type the speed in Value and choose its unit in From.
- Choose the unit you want in To. The result appears at once, and the table shows the speed in every other unit.
- Use the ⇄ button to flip the two units, for example to check a speed limit in both km/h and mph.
Speeds can only be zero or positive here, since the tool measures how fast something moves, not which direction.
Conversion factors
| Unit | Equals in metres per second | Where it is used |
|---|---|---|
| Kilometre per hour | 0.277778 m/s (1 ÷ 3.6) | Road speeds in most countries |
| Mile per hour | 0.44704 m/s (exact) | Road speeds in the US and UK |
| Knot | 0.514444 m/s (1,852 m per hour, exact) | Ships, aircraft, weather |
| Foot per second | 0.3048 m/s (exact) | Ballistics, engineering in the US |
| Mach (sea level, 15 °C) | 340.294 m/s | Aircraft speed relative to sound |
| Speed of light | 299,792,458 m/s (exact) | Physics, astronomy |
Mach is not a fixed speed. The speed of sound depends on air temperature, so Mach 1 is about 340 m/s at sea level but only about 295 m/s at cruising altitude, where the air is around −56 °C. The row here uses the standard sea-level value, so treat Mach results as approximate.
Formula and a mental shortcut
m/s = km/h ÷ 3.6 km/h = m/s × 3.6 km/h = mph × 1.609344 mph = km/h × 0.621371 1 knot = 1.852 km/h = 1.15078 mph
The number 3.6 appears because there are 3,600 seconds in an hour and 1,000 metres in a kilometre. For quick mental conversion from km/h to mph, multiply by 0.6 (accurate to within about 3.5%). Going the other way, multiply mph by 1.6. A 100 km/h limit is therefore about 60 mph, and 70 mph is about 112 km/h.
Worked examples
- 100 km/h in mph: 100 ÷ 1.609344 = 62.1371 mph.
- 60 mph in km/h: 60 × 1.609344 = 96.5606 km/h, so a 60 mph limit is just under 97 km/h.
- 100 km/h in m/s: 100 ÷ 3.6 = 27.7778 m/s, which is the distance covered in one second at motorway speed (almost 28 m, about six car lengths).
- 20 knots in km/h: 20 × 1.852 = 37.04 km/h, a brisk speed for a ship.
- A 9.58-second 100 m sprint in km/h: 100 ÷ 9.58 = 10.44 m/s × 3.6 = 37.58 km/h.
Common speeds for reference
| Speed | km/h | mph | m/s |
|---|---|---|---|
| Walking pace | 5 | 3.1 | 1.4 |
| Cycling | 20 | 12.4 | 5.6 |
| City speed limit | 50 | 31.1 | 13.9 |
| Motorway (EU) | 120 | 74.6 | 33.3 |
| Motorway (US) | 113 | 70 | 31.3 |
| High-speed train | 300 | 186 | 83.3 |
| Airliner cruise | 900 | 559 | 250 |
| Sound at sea level | 1,225 | 761 | 340 |
Why aviation and shipping use knots
The knot comes from sailors measuring a ship's speed by letting a rope with evenly spaced knots run out behind the boat for a fixed time. Today a knot is defined as exactly one nautical mile per hour, and a nautical mile is exactly 1,852 metres, close to one minute of latitude on the Earth's surface. That link to latitude and longitude is the reason navigators still use it: distance on a chart can be read straight off the scale, and speed in knots tells you how many minutes of latitude you cover in an hour.
Aircraft airspeed is also given in knots, although some jets are described in Mach number at cruising altitude, where the speed of sound is lower. Weather services use knots for wind speed over the sea. If you see a wind forecast of 25 kn, that is 46 km/h, 29 mph or 12.9 m/s.
Stopping distance and why speed matters
Speed conversions often matter for safety. Kinetic energy grows with the square of speed, so a car at 100 km/h carries about 56% more energy than one at 80 km/h, not 25% more. Braking distance follows the same rule: doubling speed quadruples the distance the brakes need, and the total stopping distance, which includes the distance covered during the driver's reaction time, rises by about three times. A car at 50 km/h (31 mph) needs roughly 24 m to stop on dry asphalt including reaction, while at 100 km/h (62 mph) it needs roughly 75 m, in line with the UK Highway Code figures for 30 and 60 mph. These are typical values and vary with the road, tyres and driver, but they show why a small change in speed matters more than it seems.
Accuracy and limits
Factors for kilometres, miles, feet, knots and the speed of light are exact definitions. Mach is shown for the International Standard Atmosphere at sea level (15 °C) and changes with temperature and altitude. The converter treats speeds as non-negative numbers and does not model relativistic effects at speeds close to the speed of light.
Frequently asked questions
How do I convert km/h to mph?
Divide by 1.609344, or multiply by 0.621371. As a quick estimate, multiply km/h by 0.6. For example, 100 km/h is about 62 mph.
What is a knot?
A knot is one nautical mile per hour, which is exactly 1.852 km/h or 1.15078 mph. It is used for ships and aircraft, and a nautical mile is based on one minute of latitude.
How fast is Mach 1?
At sea level in standard conditions, it is about 340 m/s, 1,225 km/h or 761 mph. It falls at altitude, because the air is colder.
How do I convert m/s to km/h?
Multiply by 3.6. A speed of 10 m/s is 36 km/h.
Is it exact?
The metric, mile, foot and knot factors are exact by definition. Only Mach is an approximation, because the speed of sound is not constant.